In recent years, fuel cells attract attention as dean energy conversion equipment having high efficiency. Especially, since a solid polymer electrolyte fuel cell using polymer membrane having proton conductivity as an electrolyte has a compact structure, and provides high output, and it can be operated by easy system, it attracts attention as power supplies for movement in vehicles etc. As a polymer electrolyte having proton conductivity used for a solid polymer electrolyte fuel cell, perfluorosulfonic acid derived materials including Nafion (registered trademark of DuPont) has mainly been conventionally used because it has excellent characteristics as a fuel cell. However since this material is very expensive, when a power generation system using fuel cells will become wide spread from now on, its cost may become big problems. In such a situation, development of cheaper polymer electrolyte that may be replaced with above perfluorosulfonic acid derived materials has been activated in recent years. Especially, polymers in which sulfonic acid groups are introduced into aromatic polyethers is one of the most promising materials, because such polymer have outstanding heat-resisting property and high film strength. For example, a polymer electrolyte composed of random copolymers of sulfonated polyether ketones in Japanese Patent KOHYO Publication No. 11-502249, and a polymer electrolyte composed of random copolymer of sulfonated polyether sulfone are disclosed in Japanese Patent Publication Laid-Open No. 10-45913 and Japanese Patent Publication Laid-Open No. 10-21943.
While proton conductivity will generally become higher in these materials as an introduced amount of sulfonic acid group in to polymer increases, a tendency of increase in water absorption of polymer is shown, Use of a film prepared from polymer having high water absorptivity, when it is used for a fuel cell, gives a big dimensional change caused by water existing in polymer generated during use of the cell, and thereby, internal stress formed in interface with electrode induces exfoliation of membrane, and fuel cell characteristics may possibly be deteriorated.
Moreover, strength of membrane itself also falls greatly by water absorption. Moreover, it is required for a polymer electrolyte membrane used in solid polymer electrolyte fuel cells that it should have a high energy efficiency. Therefore, it is important to reduce membrane resistance of a polymer electrolyte membrane as much as possible, and it is desired for thickness to be thinner for this reason.
However, since thin thickness inevitably reduced strength of film, when a polymer electrolyte membrane was built into a solid polymer electrolyte fuel cell or water electrolysis equipment, etc., there occurred problems that the membrane might be fractured, or after incorporating the membrane, the membrane might be broken or a peripheral sealing portion of the membrane might be torn by differential pressure of both sides of the membrane.
In order to improve a membrane strength of a polymer electrolyte membrane, a technique of combining porous membranes or nonwoven fabrics with polymer electrolyte is also proposed. For example, a cation exchange membrane in which a cation exchange resin is filled into pores of porous membrane made of ultrahigh-molecular weight polyolefin is disclosed as a polymer electrolyte membrane for solid polymer electrolyte type fuel cells in Japanese Patent Application Laid-Open No. 1-22932. Besides, a cation exchange membrane in which fluorinated derived resin represented by polytetrafluoroethylene is used as a material resin of base material is disclosed in Japanese Patent Application Laid-Open No. 6-29032, and Japanese Patent Application Laid-Open No. 9-194609.
However, these composite membranes using cation exchange resins have problems in water resistance and heat resistance property of ion exchange resins, and even if completed with porous membranes and nonwoven fabrics, sufficient characteristics as a polymer electrolyte membrane for solid polymer electrolyte type fuel cells are not demonstrated.
Besides, in the above described Japanese Patent Application Laid-Open No. 1-22932, Japanese Patent Application Laid-Open No. 6-29032, and Japanese Patent Application Laid-Open No. 9-194609, a method is disclosed in which a solution of polymer electrolyte is impregnated into pores of a porous membrane, and subsequently solvent is removed, as a method of combining porous membranes and nonwoven fabrics with polymer electrolytes. However, when penetrating power of a solvent used for solution of a polymer electrolyte to porous membranes was small, the solution was hardly impregnated into pores and the combining proved to be difficult. On the other hand, although the solution is impregnated to fill pores with electrolyte when solvent having large penetrating power is used, in case where high volatility of solvent used, temperature on surface of the film becomes lower than inside of the pores due to heat of vaporization of the solvent, and thereby dew condensation will occur on the surface or convection of solution will take place within the pores. Therefore, convection marks (sometimes referred to as dull finish) may remain or deterioration of appearance of obtained composite membrane may be observed because of dew condensation. Thus, when a composite membrane has bubble and/or unevenness, and when stress is applied, bubble and concavo-convex demonstrate function as a stress concentration point, and thereby a possible damage of the composite membrane might be induced.
Besides, a method of improving penetrating power of a solvent used for solution of a polymer electrolyte, and of obtaining composite membrane by giving surface treatment to a porous membrane is also disclosed. For example, a method to conduct hydrophilizing processing by plasma etching etc. on porous fluorinated resin film (Japanese Patent Publication No. 62-252074), a method for treating a surface of a porous membrane with surface active agents (Japanese Patent Publication Laid-Open No. 04-204522), and a method for hydrophilizing process of a porous membrane by  (with ?) a different polymer from polymer electrolyte used for membrane (Japanese Patent Publication Laid-Open No 6-271688) etc, are proposed.
However, a problem is brought about in which the surface of the porous membrane is hydrophilized by plasma etching processing given to a porous membrane, but strength of porous membrane itself falls. Besides, in a method of hydrophilizing with surface active agent or other polymers, a problem occurred that reduction of amount of polymer electrolyte impregnated in porous membrane significantly deteriorated fuel cell characteristics at time of being used as a polymer electrolyte membrane for solid polymer electrolyte type fuel cells.